The triggering of restraining devices in a motor vehicle usually occurs as a function of various parameters such as the severity of the impact and the position and weight of the passengers. In this context, it is useful to take the respective belt force into consideration as well, which is explained in greater detail in the following on the basis of an example.
For the triggering of a multi-stage airbag and for the triggering of belt tighteners, data may be acquired regarding the profile of the individual passengers from which conclusions as to their weight may be drawn. In practice, occupant-classification systems having a force-sensor array installed in the seat can be used for this purpose. The force sensors may be foil strain gauges for direct weight measurement. However, the force-sensor array may also be implemented in the form of a sensor mat having pressure-sensitive sensor cells, which are arranged in a matrix and which may be activated individually, so that the pressure distribution in the seat may be detected in the form of a value matrix. The heavier a person, the more heavily the seat material is compressed and the higher the number of activated sensor cells. Of course, the pressure value detected by a sensor cell can also depend on the respective pressurization. In evaluating the pressure data detected by the individual sensor cells of the sensor mat, the number of the activated sensor cells is therefore taken into consideration in addition to the individual pressure values. A control device, installed in the seat, carries out the evaluation. The result of the evaluation is transmitted to the air bag control device, so that the individual seat occupancy may be taken into consideration in the triggering of the restraining device. This method is largely independent of external conditions such as cold or wear.
However, conventional occupant-classification systems regularly reach their limits in the detection of child seats, in particular of strapped-in child seats, if the applied belt force is not taken into consideration.
A child seat may indeed be detected on the basis of its seat profile, which does not resemble that of an adult. However, if a child seat is strapped in very tightly, using a belt force of 150 N, for example, it is pressed into the seat at a corresponding force. In the process, the seat profile of the child seat is distorted; the child seat appears heavier than it actually is. This means that the evaluation result of an occupant-classification system may be rendered inaccurate by the application of a corresponding belt force, to such an extent that a child seat is classified as an adult and the passenger airbag is erroneously released.
However, when it is a person that is strapped in, a belt force of more than 30 N cannot really be applied over a long period of time since the belt normally yields in these cases. Furthermore, such a high belt force would be so uncomfortable to the occupant that it would not be maintained for any greater length of time. Therefore, when the belt force exceeds a certain threshold value it can be assumed with a high degree of certainty that it is not a person that is strapped in but a child seat or some other object. On the basis of the data of the applied belt force, it is thus already possible to implement an occupancy detection to a certain degree. For this reason, conventional occupant-classification systems often include a belt-force sensor in addition to a force-sensor array.